1. Field of the Invention
The present invention relates to a device for controlling an internal combustion engine, more particularly to a control device used for controlling a fuel injection or an ignition timing.
2. Description of the Related Art
In a D-J type fuel injection system for an internal combustion engine, a pressure sensor is provided in an intake passage (or a surge tank) downstream of a throttle valve of an intake system to sense an intake pipe pressure as a factor indicating a load of the engine. A flow rate of fresh air fed into a cylinder bore is obtained from the intake pipe pressure and the engine speed, so that the amount of fuel to be injected to obtain a predetermined air-fuel ratio is calculated, and the fuel injector then carries out a fuel injection.
Namely, in the D-J system, the intake pipe pressure is an important parameter when obtaining the amount of fresh air fed into the internal combustion engine, and therefore, if a change occurs in the intake pipe pressure due to exhaust gas recirculation (abbreviated as EGR below, an output value of a sensor sensing the intake pipe pressure must be corrected in accordance with the amount of EGR to obtain a correct amount of fresh air. In Japanese Unexamined Patent Publication No. 55-75548, for example, a differential pressure sensor is provided to determine a differential pressure between an upstream portion and downstream portion of a constant area orifice formed in an EGR passage, and an output value of an intake pipe pressure sensor is corrected based on a signal output from the differential pressure sensor in accordance with the amount of EGR, to obtain a correct amount of fresh air regardless of the EGR.
Therefore, in a conventional D-J type fuel injection system, the intake pipe pressure sensor senses not only the amount of fresh air but also the flow rate of gases including the EGR gas, blowby gas and the like fed into the internal combustion engine. Accordingly, the contribution of the EGR gas and blowby gas to the intake pipe pressure is sensed to correct the output value of the intake pipe pressure sensor and thereby obtain the correct amount of fresh air to be fed into the internal combustion engine. Therefore, since the amount of fresh air is not directly sensed in a conventional system, the sensing accuracy is low, and thus the internal combustion engine can not be controlled with high accuracy.
In copending U.S. patent application No. 07/151,422 filed on 2 Feb. 1988, now abandoned, the present applicant proposed a construction in which an oxygen sensor sensing a partial pressure of oxygen contained in an intake system is provided, to obtain a weight of oxygen in fresh air from the partial pressure of the oxygen, to thereby provide a more accurate control of the internal combustion engine.
The oxygen sensor provided in the above proposed construction, however, is affected not only by a partial pressure of oxygen but also by a total pressure of oxygen, so that a weight of oxygen of fresh air cannot be obtained with high accuracy, and therefore, the control of the internal combustion engine is not absolutely accurate.